With growing ecology mood, private power generation in home using such as a solar battery becomes general. However, the private power generation in home not always can generate electric power according to power consumption of the home. Therefore, electric power may be excessively generated in some hours. For this reason, an electric power trading system is allowed. In the electric power trading system, a user sells excessive electric power back to an electric power company when there is more generated power than power consumption in the home, and buys electric power from the electric power company when generated power is not enough for the home. In this regard, a power monitoring system (for example, refer to Japanese Patent Application Laid-Open No. 2008-202983) or a trading amount estimation system (for example, refer to Japanese Patent Application Laid-Open No. 2009-60720) used for the electric power trading system are proposed.
However, it is known that electric power selling may become impossible if the electric power selling is conducted on a large scale. In order to sell electric power from a home power generator via a general distribution line, the home power generator needs to supply higher voltage than present supplied voltage of the distribution line. For example, if supply voltage of the distribution line is 100V, in order to sell electric power, electric power generated by the home power generator may be converted into, for example, 101V for supplying voltage. However, in this case, if the electric power company will supply the distribution line with electric power of 103V, the electric power of the home power generator can't be sold.
In order to prevent the above condition, generally, supplying voltage of the home power generator is preliminary set relatively high. For example, supplying voltage of the home power generator is set at 105V, electric power can be sold in usual case. Primitively, voltage of a distribution line varies by a variety of factors. Therefore, it is allowed to vary the voltage within a certain range. Therefore, even if electric power is sold at higher voltage than 100V, it doesn't cause problems as long as the voltage falls within the allowed range.
However, further problem may occur when there is a plurality of sellers of electric power. Here, such a case is thought that the allowed range of variation of voltage is 100V with a margin of range of plus or minus 6V, and there are two sellers of electric power A and B in the neighborhood. As mentioned above, when a supplying voltage of a power generator of the seller A is set at 105V, and when there is low power consumption in the neighborhood, voltage of the distribution line in the neighborhood may increase to 105V by the electric power selling of the seller A. In this condition, if the seller B wants to sell electric power, the supplying voltage of a power generator of the seller B needs to be set at, for example, 106V, because the supply voltage of the distribution line is 105V. In this case, during the seller B sells electric power, the seller A becomes impossible to sell electric power. In this case, if the seller A further increases the voltage for electric power selling to 107V, for example, the voltage exceeds the allowed range, and quality of supply power is to be deteriorated.
Also, as mentioned above, voltage of the distribution line varies by a variety of factors. Voltage of the distribution line varies particularly by concern with the relation between supply and demand of electric power, that is, the relation between electric powers supplied from home power generators and consumed power at homes.
For example, there is more consumed power than electric power sold by the home power generators, voltage of the distribution line is maintained at service voltage of the electric power company. On the other hand, if there is less consumed power than electric power sold by the home power generators, voltage of the distribution line increases gradually. Therefore, when voltage of the distribution line increases to reach the voltage for electric power selling, the electric power selling becomes impossible.